Rejection of unwanted stray and background radiation falling on a detector of an imaging system can significantly improve the signal-to-noise ratio of the system. In applications where signal and background radiation are in the same waveband, such selective rejection is essential.
Terrestrial observing systems which operate in the infrared are particularly sensitive, since infrared wavelengths are emitted by surfaces surrounding the target which are at the same temperature as the target. Also, the imaging system itself and the imaging system platform emit infrared wavelengths. For space reconnaissance applications, this difficulty may also occur at ultraviolet and visible wavelengths.
The most common solution has been to cool the system to below 200K to suppress background radiation, to carefully baffle the optical train, and to isolate the instrument from all heat sources. However, the implementation of such techniques often results in unacceptable penalties in weight, volume, complexity, or cost.
For an imaging system receiving an image having only a fixed narrow frequency, the provision of a fixed-frequency narrow-band optical interference filter mounted to the detector adequately eliminates most stray and background radiation.
For spectral imaging systems, the provision of an optical filter, the transmittance of which varies linearly in wavelength with location on the filter, helps to eliminate stray and background radiation. The filter is mounted to the detector of the system, and is matched to the frequencies of the spectrum of the image such that, for each frequency, the filter transmits only the target frequency.
However, with such a transmission-frequency-varying filter, the frequency variation of the filter must be precisely matched to the frequency dispersion provided by the diffraction element of the system. As such, the filter must be mounted to, and aligned with, the other elements of the imaging system with extreme precision.
Any lateral displacement of the filter with respect to an incoming spectral signal results in a spectral mismatch such that detection of the spectral signal is severely degraded. Displacement relative to the detector causes a wavelength error in proportion to the amount of displacement.
Any possible misalignment of the filter along the spatial axis of the detector can be remedied by providing a filter having a transmission area oversized in the spatial direction. However, misalignment along the spectral axis of the detector cannot substantially exceed 0.1 detector element pitch unit before a resulting mismatch between filter transmission and dispersed signal become significant and degrades performance. Likewise, rotation between filter and detector axes produces a degrading effect, causing loss of signal and spectral dilution or blurring.
Further, any tilting of the filter with respect to the incident angle of the spectral image causes defocusing and introduces the possibility of damaging mechanical contact.
An additional problem arises from the necessity to mount the filter close to the sensitive photosurface. Space must be provided for relative movement during shock or vibration. However, space between the filter and the detector leads to the spectral mixing, i.e., crosstalk, of adjacent columns.
The spacing between separated filter/detector assemblies must be maintained at a minimum of about 50 micrometers (.mu.m)to prevent contact between the elements resulting from thermal distortions and vibration induced motion. Thus, a limit to the signal-to-noise ratio enhancement is encountered.
Each of these factors is exacerbated in a system in which the filter and detector are aligned at room temperature, then cooled to an operational temperature. If thermal characteristics of the filter and relative movement of the filter and detector during cooldown are not correctly predicted, the filter or the detector may be destroyed. In any event, to achieve proper alignment, an iterative process is employed which requires considerable time and finesse to successfully complete.
Prior art optical systems, including a detector and a filter, are found in, for example, U.S. Pat. No. 4,783,594 to Schulte et al., U.S. Pat. No. 4,939,369 to Elabd, U.S. Pat. No. 4,910,401 to Woods, U.S. Pat. No. 4,910,523 to Huquenin et al., and U.S. Pat. No. 4,940,895 to Mansfield.
The Schulte et al. and Elabd patents each disclose a system having a detector with a filter coated directly thereon. The Woods, Huquenin et al., and Mansfield patents each disclose a system with a filter mounted to a detector. However, none disclose a frequency-transmission-varying filter mounted to or coated on a detector and, thus, none are concerned with the particular problems associated with such filters.
As can be appreciated, there exists a need for an improved spectral imaging system with a frequency-transmission-varying filter which eliminates the mounting of a filter adjacent to a detector.